1. Technical Field
The present invention relates to a device and method for producing a single crystal, in particular, a device for producing a single crystal by pulling it by the Czochralski method in which the temperature distribution and thermal history of a pulling single crystal are controlled to improve the production efficiency of single crystals and the qualities thereof, and a method for producing a single crystal by using the device.
2 . Background Art
In recent years, high- integration and high- precision of semiconductor devices are more and more advanced, and wafers of semiconductor crystals are becoming larger in diameter and higher in quality. Such semiconductor crystals are mainly produced by the Czochralski method (the pulling method), in which various efforts to produce semiconductor crystals having further large diameter and high quality are made.
For example, explaining such a case that a silicon single crystal rod is produced by the Czochralski method as shown in FIG. 4, a pulling chamber (a metal chamber) 1 is provided with a quartz crucible 3 in the middle, the quartz crucible 3 being supported by a graphite susceptor 4 which is supported at the center of the bottom by a supporting shaft 4 which is rotatable and up- and- down movable. The quartz crucible is filled with a raw material, polycrystalline silicon, which is heated and melted by a graphite heater 6 surrounded with a heat insulating cylinder 5 to give melt 7. The pulling chamber 1 has an opening part 8 in the middle of the ceiling thereof, to which a sub- chamber 9 is connected. A pulling shaft 11 which is rotatable and up-and- down movable and holds a seed 10 at the end is moved down through the sub- chamber 9, thereby the seed 10 is dipped into the melt 7, and subsequently the seed 10 is pulled with rotating the pulling shaft 11 and the quartz crucible 3, thereby a single crystal rod 12 can be grown following the end part of the seed 10. The problems arising when single crystal rods are produced by such a conventional Czochralski method are as follows: First, in pulling a crystal, a protective gas such as, for example, argon gas, is introduced from the upper part of the sub- chamber 9 and discharged from a discharge outlet 15. The gas to be introduced is extremely highly pure, but in the pulling chamber a reaction between the quartz crucible 3 and the silicon melt 7 takes place so that vapor of SiO occurs from the surface of the melt 7, the vapor of SiO being present in the upper part of the inside space of the pulling chamber 7. Most of the vapor of SiO flows downward and is discharged from the discharge outlet 15, but a part thereof is deposited as aggregate of fine powders in a form of layers or masses on such places as those having relatively low temperatures such as an inside wall 16 of the upper part of the quartz crucible 3 or an inside wall 17 of the upper part of the pulling chamber 7, when any turbulent flows 13, 14 are present in the upper part of the inside space of the chamber 7 or around the single crystal rod 12 and the surface of the melt 7. The deposited SiO falls down on the surface of the melt in pulling a single crystal and attaches to the interface of growing the crystal, which results in dislocation of the pulling single crystal rod.
Furthermore, in the pulling chamber, a reaction between carbon materials such as the graphite susceptor 2, the graphite heater 6, the heat insulating cylinder 5 (for example, made of graphite felt) and the like and the above- mentioned SiO or the quartz crucible takes place so that a CO gas occurs. When the turbulent flows 13, 14 are present in the upper part of the inside space of the pulling chamber 7 and around the single crystal rod 12 and the surface of the melt 7, the CO gas flows downward and comes into contact with the surface of the melt, which results in the increase of the concentration of carbon in a pulling single crystal silicon rod and the deterioration of the characteristics of integrated circuit devices on the wafers produced from the single crystal rod.
The pulling speed of the single crystal rod 12 is dependent on a temperature gradient of the single crystal rod on the crystal- melt interface, the temperature gradient being greatly affected by the radiant heat from the graphite susceptor 2, the quartz crucible 3, the surface of the melt and the like to the crystal. In order to improve the productivity of single crystal rods, the pulling speed of the single crystal rod should be as fast as possible. However, if the diameter of a single crystal to be pulled is larger, the latent heat for crystallization as well as the above- mentioned radiant heat are increased so that the pulling speed of the crystal is decreased and the productivity thereof is also remarkably lowered. If a crystal to be pulled is larger in diameter, it becomes difficult to uniform the temperature of the entire crystal- melt interface so that the ratio of single crystallization is decreased and the yield of single crystals is remarkably reduced.
Regarding qualities of crystals, for example, in a case where integrated circuit devices are formed on a wafer of a silicon single crystal, oxidation induced stacking faults (hereinafter, referred to as OSF's), swirl defects and other microdefects are easily formed in a thermal oxidation process, which lead to the deterioration of the characteristics of integrated circuit devices. Using any conventional devices by the Czochralski method, however, it is difficult to completely suppress the generation of these defects, this tendency being more remarkable, in particular, with the diameter of crystals to be pulled becoming larger.
Moreover, the requirements for the qualities of semiconductor crystals become severer with high- integration and high- precision of semiconductor devices being advanced in recent years, so that further high purity, lower level of defect generation and uniformity of single crystals are required. In particularly recent years, it has been proved that not only the higher purity of raw materials and higher cleanness of members of a device used for the production of a single crystal and higher precision of the device but also the thermal history of a growing single crystal has a great influence on the generation of crystal defects and the like. For silicon crystals, for example, it has been confirmed that the levels of OSF's, oxide precipitates, BMD's (bulk micro-defects), FPD's (flow pattern defects), LSTD's (laser scattering tomography defects) and COP's (crystal originated particles) as well as various characteristics such as a dielectric breakdown voltage and the like are affected by the thermal history. For compound semiconductors such as GaP, GaAs, InP and the like, it has been confirmed that the dislocation density and the level of defects which function as a donor or an accepter are greatly influenced by the thermal history. Accordingly, some devices having various structures of the inside of the pulling chamber have been proposed in order to adjust the thermal history of a growing crystal to control the levels of any defects therein, but it is not possible to highly precisely control the thermal history by using such devices.
The following proposals have been made in order to solve the above- mentioned problems:
i) Japanese Patent Publication (KOKOKU) 57-40119
It describes a device for pulling a single crystal in which a crucible 3 and melt 7 therein are partly covered, as shown in FIG. 5, wherein it is provided with a member comprising a flat and circular rim 30, which is above a crucible 3 and projects from the edge of the crucible 3, and a connecting part 31, which is attached to the inside edge of the rim 30 and is in a form of a cylinder or a tapering cone, the inside height of the connecting part 31 being 0.1 to 1.2 times as long as the depth of the crucible 3.
ii) Japanese Patent Laid- Open (KOKAI) 64-65086
It describes a device for producing a single crystal rod, as shown in FIG. 6, wherein it is provided with a cylinder 19 coaxially surrounding a pulling single crystal rod 12, one end of the cylinder 19 being airtightly connected to the edge of the outlet in the middle of the ceiling of a pulling chamber and the other end thereof facing the surface of melt 7 in a quartz crucible 3 and being provided with a collar 20 which is formed by turning up outside and spreading.
These devices have such effects as the increase of the pulling speed, the suppression of SiO precipitates falling into silicon melt, the suppression of generation of OSF's, etc. to some extent. However, they are unsatisfactory under further increase in a desired diameter (in case of silicon single crystals, that are 8 inches or more) and further heightening of the desired qualities of single crystals in recent years.
That is, regarding the device under i), any measures against new requirements for qualities which become problems in recent years, such as a level of OSF's and a dielectric breakdown voltage are not taken, and in the device under i) the covering member is in fact made of relatively thin metal plate, which has a low effect of insulating a pulling single crystal from the radiant heat, so that the device under i) is not suitable for pulling single crystals having a large diameter such as 8 inches or more. Furthermore, since the connecting part 31 which surrounds a single crystal and is in a form of a tapering cone has a low inside height, the crystal, after it is not surrounded with the connecting part 31 in a pulling process, emits radiant heat directly to the metal chamber cooled by water so that it is not capable of controlling the temperature distribution and thermal history of the crystal, and the upper part of the inside space of the pulling chamber is broad so that turbulent flows 14 occur which bring about various harmful influences.
The device under ii) has the cylinder 19 coaxially surrounding a pulling single crystal rod, one end of the cylinder 19 being airtightly connected to the edge of the outlet in the middle of the ceiling of a pulling chamber and the other end thereof facing the surface of melt 7 in a quartz crucible and being provided with a collar 20 which is formed by turning up outside and spreading, so that the straightening effect of a protective gas and the effect of keeping the temperature of a crystal or cooling a crystal are improved in comparison with the device under i), but, since the collar 20 is relatively small and made of thin material, the device under ii) has also only insufficient effects of insulating the crystal from the radiant heat in pulling a single crystal having a large diameter in recent years. In the device under ii), it is possible of adjusting the thermal history of the pulling crystal to some extent by adjusting a spread angle .alpha. of the collar 20, but the range capable of adjusting the thermal history is narrow, and such highly precise and extremely minute control of the thermal history as satisfies all of various qualities of crystals which are required in recent years is not possible.
As mentioned above, since the conventional devices have only an insufficient effect of insulating a crystal from the radiant heat, the range capable of adjusting the thermal history and temperature distribution of crystals is very narrow, so that the change of design of the device for producing a single crystal including the change of the inside structure of the chamber is required every time the diameter of a crystal to be pulled is changed, this meaning a disadvantage of doing all over again.